This web page contains problems that supplement Chapter 4 on contact forces and the momentum principle, in the introductory textbook Matter and Interactions by Ruth Chabay and Bruce Sherwood. Topics include tension, normal forces, uniform circular motion, tangential forces, and 2-D and 3-D ball-and-spring models. Modern physics is introduced as students calculate approximate length and stiffness of an interatomic bond in a simple cubic lattice. Each problem can be viewed separately, with solutions, or downloaded as a pdf file. The site includes videos showing solutions to many of the problems. These can be followed with RSS or as podcasts from iTunes. Instructors are encouraged to submit their own problems and solutions to the web site. Problems should be submitted in LaTex. SEE RELATED ITEMS ON THIS PAGE to link to an article discussing the Matter and Interactions approach in detail.